1. Field of the Invention
This invention generally relates to resonant circuits, filters, and antenna duplexers, and more particularly, to a resonant circuit, filter, and antenna duplexer, in which an inductor and capacitor are connected in parallel with a resonator.
2. Description of the Related Art
In recent years, mobile telephones and mobile information terminals have become widespread rapidly, with the advancements of mobile communications systems. For example, the mobile telephone terminals communicate at high-frequency bands such as 800 MHz to 1.0 GHz and 1.5 GHz to 2.0 GHz. A device for the mobile communications system often employs a high-frequency filter having a resonant circuit or antenna duplexer having the high-frequency filter.
Referring to FIG. 1A, the resonant circuit used in the above-described device forms a one-port resonant circuit, in which a resonator S21 is arranged between an input terminal In and an output terminal Out. A Surface Acoustic Wave (SAW) resonator or Film Bulk Acoustic Resonator (FBAR) is employed as a resonator. FIG. 1B is a top view of a SAW resonator. Mounted on a piezoelectric substrate 70 are: comb-like electrodes IDT (Interdigital Transducer) respectively connected to the input terminal In and the output terminal Out; and reflectors R0 provided at both sides of the comb-like electrodes IDT. The comb-like electrodes IDT and the reflectors R0 are made of a metal such as aluminum (Al), for example. Here, in the accompanying drawings, it is shown that the reflectors R0 and the IDT have electrode fingers less than the actual number of the fingers.
FIG. 1C is a top view of FBAR. FIG. 1D is a cross-sectional view thereof. A lower electrode 75, a piezoelectric film 74, and an upper electrode 73 are deposited on an opening 76 in a substrate 72 (an example is a silicon substrate). Aluminum nitride, for example, is used for the piezoelectric film 74. Instead of the opening 76, a multilayer reflection film is provided in some cases.
A ladder type filter, in which one-port resonant circuits are connected in series and in parallel, is used for a high-frequency filter. FIG. 2 shows a configuration of the ladder type filer. Between the input terminal In and the output terminal Out, series resonators S11 and S12 are connected in series and parallel resonators P11 and P12 are connected in parallel. Referring to FIG. 3A through FIG. 4B, the operation principle of the ladder type filter will be described. The ladder type filter can be separated into series resonant circuits and parallel resonant circuits. Referring to FIG. 3A, in a series resonant circuit, assuming that a resonator S21 is a one-port resonant circuit, one of two signal terminals is set to the input terminal In and the other terminal is set to the output terminal Out. Referring to FIG. 3B, in a parallel resonant circuit, assuming that a resonator P21 is a one-port resonant circuit, one of the two signal terminals is connected to a ground terminal and the other terminal is connected to a short-circuit line.
FIG. 3C shows passband characteristic from the input terminal In to the output terminal Out of the series resonant circuit and parallel resonant circuit. The horizontal axis represents frequency, and the vertical axis represents band-pass amount. The passband characteristic of the series resonant circuit is indicated by a solid line, and that of the parallel resonant circuit is indicated by a dashed line. The passband characteristic of the series resonant circuit includes one resonance point (resonance frequency) frs and one antiresonance point (antiresonance frequency) fas. The passband amount is the highest at the one resonance point frs, and is the lowest at the one antiresonance point fas. On the other hand, the passband characteristic of the parallel resonant circuit includes one resonance point frp and one antiresonance point fap. The passband amount becomes the lowest at the one resonance point frp, and becomes the highest at the one antiresonance point fap.
FIG. 4A shows a structure of a one-stage ladder type filter. Referring to FIG. 4A, a series resonator S22 serving as a series resonant circuit is connected in series between the input terminal In and the output terminal Out, and a parallel resonator P22 serving as a parallel resonant circuit is connected between the output terminal Out and the ground terminal. At this point, it is designed that the resonance point frs of the series resonant circuit is substantially identical to the antiresonance point fap of the parallel resonant circuit. FIG. 4B shows passband characteristic from the input terminal In to the output terminal Out of the one-stage ladder type filter. The horizontal axis represents frequency, and the vertical axis represents band-pass amount. With the structure shown in FIG. 4A, the passband characteristic of the series resonant circuit and that of the parallel resonant circuit are combined, and the passband characteristic of FIG. 4B is obtainable. The band-pass amount is the highest around the resonance point frs of the series resonant circuit and the antiresonance point fap of the parallel resonant circuit, and is the lowest at the antiresonance point fap of the series resonant circuit and the resonance point frp of the parallel resonant circuit. The passband is a frequency range that ranges from the resonance point frp of the parallel resonant circuit to the antiresonance point fap of the series resonant circuit, and the attenuation range is the frequency range equal to or lower than the resonance point frp of the parallel resonant circuit and equal to or higher than the antiresonance point fap of the series resonant circuit. In this manner, the ladder type filer functions as a band-pass filter.
There has been proposed an antenna duplexer with the use of a filter having the above-described resonator. The antenna duplexer employs two band-pass filters to arrange a transmit filter between the transmitting terminal and the antenna terminal and arrange a receive filter between the receiving terminal and the antenna terminal. A matching circuit (for example, phase shifter) is also arranged between the antenna terminal and the transmit filter or between the antenna terminal and the receive filter. The antenna duplexer has functions of outputting a transmitting signal input from the transmitting terminal, from the antenna terminal, and outputting a received signal input from the antenna terminal, from the receiving terminal.
A description is given of the functions of the matching circuit in a case, for example, where the matching circuit is arranged between the antenna terminal and the receive filter. The matching circuit has functions of preventing the electricity of the transmitting signal input from the transmit terminal from entering the receive filter, and outputting the transmitting signal from the antenna terminal. Generally, the impedance equals almost zero at the receive filter in the frequency band of the transmitting signal. Therefore, a large part of the electricity of the transmitting signal enters the receive filter. So, the matching circuit is provided to convert the impedance in the frequency band of the transmitting signal at the receive filter into almost infinite.
As disclosed in Japanese Patent application Publication No. 2003-332885, Japanese Patent application Publication No. 2003-69382, Japanese Patent application Publication No. 2004-135322, and Japanese Patent application Publication No. 2004-242281, there has been proposed the ladder type filter in which an inductor is connected in parallel with a resonator. FIG. 5 shows a conventional filter according to the above-described conventional techniques. The series resonators S11 and S12 are connected in series between the input terminal In and the output terminal Out, and the parallel resonator P11 is connected between nodes of the resonators S11 and S12 and the ground. The parallel resonator P12 is connected between the output terminal Out and the ground. In addition, inductors L11 and L12 are respectively connected in parallel with the series resonators S11 and S12. With such configuration, two antiresonance points of the series resonant circuits are obtainable. Thus, with the use of the two antiresonance points, it is possible to provide a filter of excellent attenuation properties.
The filter that includes a SAW resonator or FBAR serving as a resonator has similar functions (double resonance characteristic) with the resonance point and antiresonance point, which has been explained with reference to FIG. 3A through FIG. 4B. Herein, “resonator” simply denotes the resonator having the double resonance characteristic such as the SAW resonator or FBAR. Also, “resonant circuit (one-port resonant circuit)” denotes a circuit that includes a single resonator or a resonator with which an inductor or capacitor is connected in parallel.
In the conventional techniques, however, the inductor connected in parallel with the resonator has a large size, and this causes a problem that the sizes of the filter and the duplexer cannot be reduced. In addition, two antiresonance points cannot be arbitrarily set. For these reasons, there is another problem in that design flexibility is degraded in the filter that utilizes the two antiresonance points.